Factors Affecting Iodine Content in Dairy Cow’s
Milk – a Review
M. A. Norouzian
1*and
F. Azizi
21
Department of Animal Science, College of Abouraihan, University of Tehran, Iran.
2
Endocrine Research Center, Research Institute for Endocrine Sciences, Shahid Beheshti University of Medical Sciences (M.C.), Tehran, Iran.
Authors’ contributions
This work was carried out in collaboration between authors. All authors read and approved the final manuscript.
Received 24th October 2012 Accepted 15th April 2013 Published 3rd May 2013
ABSTRACT
Recently, milk has been suggested as one of the main sources of iodine for human intake. However, there is the potential for problems resulting from excessive amounts of iodine in cow milk. Supplementary feed iodine increases milk iodine and is apparently the cause of many of the high milk iodine values. Iodine teat dips and sanitizers also contribute to increased milk iodine values. Variability of these practices may result in variation of iodine concentrations in milk. In addition, the transportation and processing phases of milk production, where iodophor sanitizers are used, season, geographic region, stage of lactation and iodine antagonists in feeds, may influence milk iodine concentration. It seems that Iodine concentration in milk should be monitored, regularly.
Keywords: Milk iodine teat dipping; season; pasteurization; stage of lactation; rapeseed.
1. INTRODUCTION
Iodine is an essential trace element for humans and animals. It is incorporated into the thyroid hormones that have multiple functions as regulators of cell activity. Iodine deficiency affects reproductive capacity, development of brain and progeny as well as growth.
Iodine occurs in nature as iodide and iodate. Its mineral forms occur ubiquitously in igneous rocks and soils, most commonly as impurities in saltpeter and natural brines. It is liberated by weathering and erosion, and, because of its water-solubility, it leaches by rainwater into surface water, the sea and the oceans, soils in mountainous regions becoming low in iodine. In many areas of the world the surface soil becomes progressively poorer in iodide through leaching processes [1]. Iodides in the sea accumulate in seaweeds, sea fish and shellfish. The major natural sources for animals (and humans) are iodides in feed (and food) and water which rarely contains enough iodine to meet animal and human’s needs [2]. However iodine deficiency is not as likely to occur today as it did several decades ago. In fact, it has been suggested that iodine toxicity is more likely to occur than iodine deficiency [3]. The increased iodine content of food is due to greater usage of iodine in food processing, an increase in the iodine content of farm animal rations, and increased use of iodine as antibacterial agents both on and off the farm. On the other hand, in today’s nutritional situation there is the potential for problems resulting from excessive amounts of iodine in human diets to cause problems [4]. Currently whole milk and dairy products contribute the highest proportion of iodine to the human diet [5,6]. A survey of iodine content in 18 brands of cows’ milk was performed between 2001 and 2002 in the Boston area, Pearce et al. [7] showed that the average iodine content of milk was 110 µg per cup. The recommended intake for adequate calcium nutrition of 4 cups daily would provide 440 µg iodine daily. This intake is 4.8 times the recommended intake for children and 2.9 times the recommended intake for adults. A combination of 4 cups of milk and two slices of bread could result in a daily ingestion of greater than 1700 µg iodine, or 11.3 times the recommendations and 600 µg above adult tolerable limits [7].
As a result, the dairy industry in many countries has looked into the potential sources of iodine in milk in order to prevent over consumption of this trace element. There are various factors for increased iodine concentration in dairy cow milk. This review discussed factors that were affected iodine content of dairy cow milk at farm and processing unit levels.
2. EFFECT OF IODINE INTAKE: SOURCE AND LEVEL
Iodine content of milk can be increased by a number of different sources. Natural feed sources have been shown to vary considerably in iodine level and will differ greatly by areas of the country and world. There were also marked differences between crops with corn silage having lower iodine content per unit of dry matter than alfalfa. In investigations using inductively coupled plasma- mass spectrometry (ICP-MS) the iodine concentrations of barley and wheat were undetectable (<20 µg/kg) [8]. Low concentrations of iodine (6µg/kg) were detected in durum wheat (Triticum durum) flour by ICP-MS [2] and neutron activation analysis (3 and 9 µg/kg) [9]. Iodine concentrations in the range 2–30 µg/kg (mean 6.1, median 4.6) were detected in cereal grains from 38 different sites in Austria using neutron activation [10]. Dermelj et al. [9] found concentrations of 15 and 19 µg/kg in two samples of soybean meal feed.
Source of iodine supplementation can be effective on iodine content of milk. In study of Miller and Swanson [14], the dairy cows group receiving potassium iodide (KI) had higher serum but lower milk iodine levels than the group receiving the same amount of iodine in the form of ethylenediamine dihydroiodide (EDDI). This demonstrates different metabolism rates or pathways for these two forms and would indicate different limits for different forms of iodine if a limit on milk iodine were to be set.
Also milk iodine concentration is related to the level of iodine consumed by dairy cow. In the dose–response experiment, five Holstein cows in the last lactation third (7–9 months after the calving) were fed increasing amounts of iodine 15, 65 and 132 mg iodine/cow/day, increasing dietary iodine intake significantly increased the iodine content of milk. A fourfold increase in iodine supplementation from 15 mg iodine/day to 65 mg iodine/day resulted in a 3.5-fold increase in milk iodine concentration. Doubling the daily iodine dose from 65 mg iodine/day to 132 mg iodine/day resulted in a further 2.3-fold increase in milk iodine concentration. Thirty to forty percent of the ingested iodine was transferred into the milk in the three groups given supplemental iodine [15].
Our data indicate that when I intake is at the recommended feeding amount of 0.534 mg/kg DM [16], milk I will be 162 µg/L. When supplemental I added to a low I diet was increased from 11 to 62 mg/cow/day (corresponding to NRC recommended level or 2.5 mg/kg DM diets), milk I concentration was raised by 3.3 times. However, when supplemental I was increased from 62 to 165 mg/cow/day (corresponding to 2.5 or 7.5 mg/kg DM diets), milk I increased only 12%. This suggests that milk I response curves should not be extrapolated linearly for I intakes. However the exact mechanisms for this reduction in secretion ratio at higher level of I supplemental are not clear; a possible explanation might be that mammary gland of dairy cow act as bio-regulator for iodine excretion [17]. Also, Miller et al. [18] noted that cows secrete less iodine into milk than most other species and have an efficient iodine recycling system via the gastrointestinal tract.
Iwarsson [19] estimated I intake and milk I output throughout 40 week lactations and found secretion ratios averaged 20.5% when milk yield averaged 16.5 kg/day. Secretion ratios in our experiment were 21.9% for all cows with mean daily milk yields of 27.5 kg during experimental period [17].
3. IODINE ANTAGONISTS IN FEEDS
A main dietary influencing factor on iodine metabolism seems to be the application of anions which inhibit the sodium iodide symporter. This active transport system extensively investigated in the thyroid also seems to allow iodine accumulation in other extrathyroidal tissues like the mammary gland [20]. The most important inhibiting anions in animal diet are thiocyanates, a degradation product of the glucosinolates (e.g. in rapeseed feedstuffs). Since the uptake of iodine in the thyroid and the mammary gland is considerably reduced by thiocyanates feeding [20, 21] rapeseed also ought to influence the blood, urinary and milk iodine. The mammary-gland-epithelial cells contain peroxidase enzymes. Iodide (Iି) is taken up via the iodide sodium symporter on the basal membrane of the epithelial cell [22]. Iି is
the proportion of total serum iodine which is nonhormone iodine [24]. Higher urinary iodine occurs in dairy cows fed rapeseed meals [25]. In contrast, the milk iodine concentration was reduced by rapeseed glucosinolates.
According to Laarveld et al. [26] feeding low-glucosinolates diets resulting in the daily intake of 14 to 47 mmol glucosinolates did not change milk iodine content while feeding low- glucosinolates rapeseed meal at higher levels in dairy cows (intake of 37–63 mmol glucosinolates /day) reduced milk iodine content.
In study of Veselý et al. [27], the daily intake of 50.14 mmol glucosinolates resulted in a significant decrease in iodine concentration in milk (196.7 µg/l for diet containing glucosinolates in comparison with 367.0 µg/l in control diet). This reduction by feeding glucosinolates had reported by other researchers [8, 25].
4. ORGANIC OR CONVENTIONAL MILK
Although conventional milk is the usual choice, organic milk is increasing in popularity because of perceived health and environmental benefits [28]. Due to the strict organic farming regulations that govern the use of mineral supplements in livestock feed [29], organic milk may contain lower concentrations of trace minerals, thereby reducing or even reversing the potential health benefits. Studies in Europe have shown conventional milk to have a higher iodine concentration than organic milk [28, 30, 31]. Iodine concentrations of the Norwegian [31] and Danish [30] summer organic milk samples were 31.8 and 40.8% lower than those of conventional milk, respectively. In UK this difference reported 42.1 % [28].
The lower iodine concentration in organic milk can be explained by differing practices on organic and conventional farms. Organic farming regulations do not allow the routine use of vitamin and mineral preparations [29]. Regulations also stipulate that at least 60% of the feed on organic farms must be fresh or conserved forage, thus limiting the use of concentrates and relying on soil minerals, which can be low in some areas. Due to these restrictions, deficiencies in some minerals, including iodine, can occur in organically farmed livestock [32]. Nitrogen-fixing crops, such as clover, are important in organic farming and are used in place of artificial fertilizers [33]. While clover contains cyanogenic glucosides [34] that are thought to exhibit goitrogenic properties and, as suggested by Rasmussen et al. [30], greater use of goitrogenic feed could lower milk-iodine concentration through the inhibition of the sodium-iodide symporter in the mammary gland of the cow.
5. TEAT DIPPING AND UDDER WASH
Iodine is commonly used as the key ingredient of pre- and post-milking teat disinfecting solution. Teat-dipping practices showed associations with iodine levels in milk; however, the effect appeared to be modulated by the way in which the sanitizer was applied. Borucki Castro et al. [35] reported that the iodine concentration in milk was not affected by dipping teats in 1% iodine sanitizer after milking. However in other studies, the effect of post-milking iodine teat dipping ranged from no effect [36] to an increase of 184 µg/L [37]. Post-dipping with 0.3% [38] and 0.5% [39] iodine increased milk iodine between 33 and 54 µg/L.
Conrad and Hemken [40] reported that teat dipping of only the right udder half with an iodine- based sanitizer resulted in similar iodine increases in both left and right udder halves.
The primary mode by which post-milking teat dipping affects milk iodine concentration is absorption from the teat’s surface and secretion into milk rather than contamination from the skin’s surface. Spraying requires about 50% more disinfectant than dipping to achieve the same degree of teat cover [41]. In addition, a larger skin area is covered with the sanitizer when the teats are sprayed rather than dipped, increasing the absorption area. Finally, the skin of the teat is washed at each milking, whereas the skin of the mammary gland is not routinely washed, leaving more time for residues to be absorbed. These factors probably explain why spraying 1% iodine post-dip dramatically increased the iodine in milk. These results suggest that automatic teat disinfection systems, which use more disinfectant and cover a larger skin area, would increase the milk iodine level even further.
Galton et al. [42] and Rasmussen et al. [43] reported that dipping with 0.5% iodine pre-dip solutions followed by complete drying of the teat did not significantly increase milk iodine content. However, pre-dipping with 1% iodine post-dip solution increased milk iodine content [43]. The higher iodine content of the post-dip solution probably plays a role, given that Galton et al. [44] reported an increase in milk iodine with a 1% iodine sanitizer but not with 0.1% sanitizer. In another study, Rasmussen et al. [43] observed greater milk iodine concentrations with a 0.25% than with a 0.5% iodine post-teat dip. These authors attributed this surprising result to the fact that the 0.5% teat dip was easier to remove, leaving less residue on the teat skin. Galton et al. [42] reported that installing the milking unit without drying the teats after dipping in 1% iodine sanitizer increased milk iodine by 600 µg/L above the level observed when teats were dried before milking.
Few studies focused on the potential effect of iodine udder wash on iodine concentration in raw milk. Cantor and Most [45] did not detect a significant increase in milk iodine levels when the compared the use of pre-milking iodine udder wash to a non-iodophor product. On the other hand, tow other studies performed by Dunsmore and Nuzum [46] and Hemken et al. [47] reported increases 35 µg/L in mean milk iodine concentration from cows washed with an iodine-based product.
6. STAGE OF LACTATION AND MILK YIELD
Although Lamand and Tressol [48] and Moschini et al. [49] did not find a correlation between stage of lactation or age of animal and milk iodine levels, other studies have reported more definitive result [19, 50]. Franke et al. [51] showed that milk iodine levels increased with the stage of lactation. Iwarsson [19] found statistically significant changes in iodine levels with the progression of lactation. In this study, during weeks 1 to 5 of lactation, the iodine concentration ranged between 5 and 30 µg/L. For weeks 36 to 40, the milk iodine levels were between 35 and 115 µg/L. It seems that this increase in iodine concentration with the progression of lactation is due to decrease of milk yield. As Iwarsson [19] found a negative correlation between iodine concentration and total milk yield. This reinforces the idea that milk yield does play a role in secretion of iodine into the milk.
7. REGION AND SEASON
milk in Great Britain, based on monitoring of 30 regions performed in 1990 and 1991, was 150 (range 40 to 310) µg/L compared to 230 µg/L in 1977-1979 [53].
Monitoring of iodine levels in raw milk purchased by 28 Bavarian dairy plants showed a content of 115 µg/L (from less than 100 µg/L in South Bavaria to over 150 µg/L in North Bavaria) [54]. Monitoring of the iodine content of milk in Ireland showed that it averaged 139 µg/L (range 2 to 435 µg/L) [55].
Variable use and variable levels of use of iodophors and ethylenediamine dihydroiodide (EDDI) and veterinary medication are primarily responsible for the variation of iodine concentrations in milk observed among countries, among regions within countries, and among dairy farms within regions.
A few studies indicated seasonal variation in the iodine content of milk. A study of 50 herds with 33 to 293 cows demonstrated that 38% of the herds had low iodine status in the spring and 70% had low iodine status in the autumn [56].
Monitoring studies of the iodine content of fluid milk published in the USA showed milk iodine content was 190µg/L milk in the summer season and 270µg/L milk in the winter season [50].
The iodine content of Norwegian milk was 88µg/L (range 63 to 122µg/L) in the summer and 232µg/L (range 103 to 272 µg/L) in the winter [31]. A study of milk iodine content in Finland showed 280 µg /L in summer and 400 µg /L in winter [57].
Seasonal variations in the iodine content of milk are usually attributed to the proportion of pasture feeding versus the use of prepared feed containing supplemental iodine. Pasture feeding may predominate in the summer, prepared feed may predominate in the winter, and spring and autumn feeding may be a mixture of both. The length of each season is influenced by the climate of the geographical location [50].
Other authors [58,59] noted that higher concentrations of iodine in winter months might reflect decreased milk production that would account for a higher concentration of iodine in the milk.
8. PROCESSING THE RAW MILK
Increased iodine concentration in milk can occur due to processing when there is inadequate drainage of milk lines following cleaning or contamination of the milk contact surface [60]. Povidone iodine disinfecting solutions are used to clean tanker trucks, vats, and milking equipment and residues from these solutions add a substantial amount of iodine to the milk.
There are few studies noted that heating and boiling of milk in pasteurizing process reduce milk iodine concentration. Magee and Glennie [61] found 20 and 26% decrease of iodine content of milk in pasteurization and boil point, respectively. In our study we found 27.1±7.1% decrease of iodine after high temperature short time (HTST) pasteurization (Fig. 1) [6].
As well, it has been reported that spray and roller drying of milk reduce iodine concentration by 40% [60]. Finished fluid milk has been found to be lower in iodine concentration that bulk tank raw milk. This could be due to a dilution effect when large volumes of milk are pooled [60].
On the other hand, a study by Aumont [62] reported that further processing including pasteurization, UHT pasteurization, and spray drying had no impact on finished milk iodine concentration compared to the raw milk iodine levels.
Fig. 1. The effect of different levels of potassium iodide and pasteurization process on milk iodine concentration
9. CONCLUSION
In recent years, the iodine content of milk increased, considerably and the amount reported in a liter of milk for some samples exceeds the safe limit as set by Food and Nutrition Board (1970). Iodine added to the ration seems to be a main source for farms with very high concentrations of iodine. Iodine teat dips and iodine sanitizers also increase the amount of milk iodine. It is particularly topical to control supplementation of feed rations with the additives containing iodine according to the physiological requirements and production conditions. It seems to be suitable to keep at systematic general monitoring of iodine content in fresh cow milk with the possibility of feedback to the agribusiness. Also, with continued research on factors affecting iodine concentration in milk as well as on-going education of producers, the dairy industry will be able to continue to ensure consumers a safe and healthy source of essential nutrients.
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